Hydrcxamic acids can be represented by the structural formula R.sub.1 C(O)N(OH)R.sub.2, where R.sub.1 is typically hydrogen or a hydrocarbon radical such as an alkyl radical, a cycloalkyl radical or an aromatic radical and R.sub.2 can be a hydrogen atom or a hydrocarbon radical such as an aromatic radical or an alkyl radical. As disclosed by Lucid in U.S. Pat. No. 3,821,351, Coleman et al. in U.S. Pat. No. 4,741,887 and Monzyk et al. in U.S. Pat. No. 4,975,253, such hydroxamic acids are useful as chelants for extracting metals, e.g. from an aqueous solution of a mixture of metals. Hydroxamic acids containing at least about 7 carbon atoms are especially suitable for metal extraction being substantially insoluble in aqueous solutions and highly soluble in industrial organic solvents, e.g. kerosene.
Many hydroxamic acids exhibit distinct extraction profiles within a narrow band of pH for different metal species that allows for selectivity during extraction and/or stripping of metals. In a typical application a kerosene solution of such hydroxamic acid intimately mixed with an aqueous solution of a mixture of metals which transfer from the aqueous phase into the organic phase as complexes of the hydroxamic acid. Specific metal species can be selectively stripped from the kerosene phase by mixing the metal-loaded extractant with a series of aqueous solutions having a pH matching the distinct extraction profile of each metal species. The commercial utility of such hydroxamic acids also depends to a large extent on their hydrolytic stability, i.e. the resistance of the hydroxamic acid to dissociate in the aqueous solutions into hydroxylamine and carboxylate species, e.g. at an extreme pH typically used for stripping complexed metal from the hydroxamic acid. Hydroxamic acids that are hydrolytically unstable, e.g. useful for only a single, or at most a few, extraction/stripping cycles, are essentially of no commercial utility. In this regard Coleman et al. have shown that hydrolytic stability of hydroxamic acids depends of the nature of the R.sub.2 radical. For instance, hydroxamic acids having hydrogen or an aromatic radical such as a phenyl group are substantially hydrolytically unstable, especially when exposed to low pH aqueous solutions commonly used to strip metal from hydroxamic acid complexes. Coleman et al. also demonstrated superior hydrolytic stability for hydroxamic acids where the R.sub.2 radical is an alkyl group, e.g. a methyl group; such hydroxamic acids are referred to in the following description of this invention as N-alkyl, e.g. N-methyl, hydroxamic acids.